The objective of the project is to continue investigations carried on since 1950 concerning Echinococcus species (Cestoda:Taeniidae) and the diseases caused by them in man (hydatid disease, sensu lato) . Principal objectives are to distinguish the larval stages of four species of Echinococcus as they occur in human organs by means of morphologic characters not previously so applied; to evaluate biologic differences between the two "strains" of E. granulosus; to investigate cytogenetic methods as a further means for distinguishing species of Echinococcus; to assess the effect of mebendazole on the larval stage of E. multilocularis and E. Vogeli in situ in experimental animals infected per os; and to describe the post-embryonic development of E. vogeli in the intermediate host and define its pattern of growth and mechanism of pathogenesis.